A habit she had to retire when she adopted two children, I suppose.
You could make arguments for or against the appropriateness, but consider:

A) They were raised to that point by others; Katherine did not get to raise them to consider nudity normal or natural from birth. Castela was also at an age when explaining the difference between taking off your clothes at home and taking them off in public would be… problematic, and with frequent “mistakes”.

B) A prospective adoptive parent is under far more scrutiny by various agencies than natural parents. “I walk around the house nude all the time” probably sends up red flags with adoption agencies, regardless of how harmless in the correct context it may be.

C) When raising two very unique young paras, with the goal of having them able to fit into normal human society at least well enough not to rile up a torch & pitchfork mob… forcing them to become involved with your personal alternative lifestyle (nudism) is not to their benefit. They have enough challenges already.

And probably the most significant: D) Having some or all parties in Kath’s home be nude all the time (2/3 of which are minors) would make writing and drawing all the home scenes of the new family extremely challenging for Paul, and would limit the nature of stories he could tell.